1. Field of the Invention
The present invention relates to a system and method for improving the performance of an on-demand routing protocol in a wireless network.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at a first node to communicate simultaneously with several other nodes in its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, now U.S. Pat. No. 7,072,650; in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, now U.S. Pat. No. 6,807,165; and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, now U.S. Pat. No. 6,873,839, the entire content of each being incorporated herein by reference. Internet Engineering Task Force (IETF) has charted a working group called Mobile Ad Hoc Networks (MANET) to standardize IP routing protocol functionality suitable for wireless routing application within both static and dynamic topologies with increased dynamics due to node motion or other factors. Two of the On-Demand (or Reactive) routing protocols considered by the group are the Ad hoc On-Demand Distance Vector (AODV) (RFC 3561) Routing Protocol and the Dynamic Source Routing (DSR) (currently an internet draft) protocol.
On-demand routing protocols create routes only when desired by the source node. When a node requires a route to a destination, it initiates a route discovery process within the network. This process is completed once a route is found or all possible route permutations have been examined. Once a route has been established, it is maintained by some form of route maintenance procedure until either the destination becomes inaccessible along every path from the source or until the route is no longer desired.
Typically the Route Discovery involves broadcasting and forwarding of Route Request (RREQ) packets until the route is found. These Route Request packets typically contain the IP address of the node originating the Route Request and a number called the Route Request ID (or RREQ ID). Each node maintains only one RREQ ID, which is incremented by one from the last RREQ ID used by the node before initiating the new RREQ packet. The RREQ ID uniquely identifies the particular RREQ when taken in conjunction with the source node's IP address. In other words, the tuple, <source node's IP address, RREQ ID>, uniquely identifies a particular RREQ in the network. When a node receives this RREQ, it checks to determine whether it has received a RREQ with the same Originator IP Address and RREQ ID within some last period of time. If such a RREQ has been received, the node silently discards the newly received RREQ. In this way, when the node receives the packet again from its neighbors, it will not reprocess and re-forward the packet.
This scheme was originally developed to reduce the overhead and processing time associated with the RREQ packet if it is reprocessed and is suitable if the routing metric used is the number of hops in this desired route. However, this scheme can lead to un-optimal routes if the protocol uses some other metric (such as link quality, delay, or throughput), as demonstrated in the Detailed Description below where a possible better route is not selected because each of the nodes discard a rebroadcasted route request if it already has seen the same tuple. The optimal route can thus never be found with the rules specified in the AODV protocol draft, especially when small routing messages can successfully be transmitted over bad links.
Accordingly, a need exists to locate and identify optimal network routes, to thus improve overall performance of a network, including improvement of throughput, delay, and packet completion rate. The present invention provides improved system performance of a wireless network by enabling nodes following an on-demand routing protocol to process (and possibly reply to) route request messages multiple times based on the routing metrics.